1. Field of the Invention
The present invention relates to the fabrication of electronic circuit packages and in particular to methods, and mechanical means for attaching electronic components, especially leadless ceramic packages for electronic devices, to each other or to supporting substrates such as circuit boards.
2. Prior Art
The microelectronics industry is steadily moving toward the use of smaller electronic circuit devices giving rise to the need for smaller and smaller connecting devices and mounting means for the circuit devices. Circuit devices, such as integrated circuits of complex nature, are embodied in large chips and chip carrier packages (CCP) which have connection pads on the faces and/or edges of the packages. Chip carrier packages can be produced with leads attached (leaded) or they can be leadless.
Leaded CCP's can be soldered directly onto printed circuit boards or printed wire boards. Leadless CCP's can be soldered onto ceramic boards or installed into connectors. However, with glass/epoxy printed circuit boards or other printed wiring boards (hereinafter substrates) leadless CCP's are usually mounted into connectors which are, in turn, mounted on the substrates because of the effect of different thermal expansion coefficients of the materials involved when subjected to temperature fluctuations. Specifically, the CCP and the substrate exhibit different thermal characteristics leading to in-plane stress and strain therebetween as a result of thermal mismatch. Also, any flexure of the substrate result in out-of-plane stresses. Prior art techniques have used small solder spheres or solder paste to attach packages to substrates. Although such known techniques have been used for such surface mounting, there exists a need for a compliant solder joint to compensate for the large stresses resulting from thermal mismatches between the CCP and the substrate, as well as from board warpage and flexure.
Different solutions have been proposed for the foregoing problems. The proper positioning of a predetermined quantity of solder may be achieved with the use of solder preforms spaced on a carrier template in the locations corresponding to the points where the solder joints are to be formed. Examples of this technique may be found in U.S. Pat. Nos. 3,320,658, issued to Bolda et al; 3,396,894, issued to Ellis; 3,472,365, issued to Tiedema; 3,719,981 issued to Steitz; 3,744,129, issued to Dewey; 4,209,893, issued to Dyce et al; and 4,216,350, issued to Reid.
Although the foregoing techniques provide for the correct placement of a predetermined quantity of solder or other suitable joint-forming material, and with the proper dimensioning of the carrier or template, sufficiently small quantities of solder can be positioned on close spacing between centers, these proposals do not address the problems of high shear strains in the solder joints.
In U.S. Pat. No. 4,412,642 to Fisher, leadless chip carriers are converted to "cast-leaded chip carriers" by molding high melting point leads to the chip carrier. Additional examples of methods and devices for soldering terminals to a printed circuit board are shown in U.S. Pat. No. 3,926,360 to Moister and in the IBM Techical Disclosure Bulletin, Vol. 21, No. 6, dated November 1978.
While the above disclosures have addressed the problem of connections which must be able to withstand the stresses from thermal cycles, none discloses a satisfactory solution which both solves the problem and is suitable for reliable manufacturing processes.
Because of the significant problems associated with the attempt to position preferred material such as a solder and to effect electrical contact, alternative less desirable conductive materials and/or mechanical approaches have been developed. U.S. Pat. No. 4,064,623 to Moore shows an electrical connector utilizing conductive rubber rods. U.S. Pat. No. 4,295,700 to Sado discloses a similar press-contact type interconnector utilizing elastic material having anisotropical electroconductivity. U.S. Pat. No. 3,991,463 to Squitieri et al, discloses a method of forming an interconnector having a row of electrically conductive flexible plastic strips. U.S. Pat. No. 4,027,936 to Nemoto et al discloses a connector having electroconductive rubber terminals. U.S. Pat. No. 4,144,648 to Grovender, discloses a connector utilizing conductive elastomer medium. U.S. Pat. No. 4,402,450 to Abraham et al, discloses contact pads of a device which are adapted for bonding components such as contacts of a circuit assembly thereto.
Unfortunately, none of the above disclosures provide the superior electrical interconnection accomplished by solder. It would therefore be highly desirable to have a solder preform delivery system with a precise placement of a predetermined quantity of material in a preferred geometric preform for the formation of a solder-type connection.
Commonly assigned U.K. patent application GB No. 2,142,568 A to Allen et al, equivalent to U.S. Ser. No. 509,684 filed June 30, 1983, now U.S. Pat. No. 4,664,309 and U.S. Ser. No. 610,077 filed May 14, 1984 the dislosures of which are incorporated herein by reference, discloses a chip carrier mounting device which includes a retaining member having a defined pattern of apertures in which are positioned preforms of joint-forming material such as solder or conductive elastomer. The instant invention is an improvement to that mounting device wherein the problem of solder preform positioning and solder preform configuration are addressed.